Production of formaldehyde from CH4 and H2 S

ABSTRACT

A method wherein a sour natural gas stream can be treated to produce formaldehyde from methane and methyl mercaptans, (primarily methanethiol (CH 3  SH) and a small amount of dimethyl sulfide (CH 3  SCH 3 )) from hydrogen sulfide, and the methyl mercaptans preferably are passed in contact with a catalyst comprising a supported metal oxide or a bulk metal oxide in the presence of an oxidizing agent and for a time sufficient to convert at least a portion of the methyl mercaptan to formaldehyde (CH 2  O), and sulfur dioxide (SO 2 ).

This application claims the priority benefits from the U.S. provisionalapplication Serial Number 60/060,171 filed Sep. 26, 1997.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention broadly relates to a process for producing usefulchemicals, and especially formaldehyde, from methane (CH₄) and hydrogensulfide (H₂ S), and especially from a gas stream containing a mixture ofCH₄ and H₂ S. More particularly, this invention provides a methodwherein hydrogen sulfide, generally separated from a gas streamcontaining methane and hydrogen sulfide, is combined with a carbonoxide, wherein the carbon oxide is selected from carbon monoxide (CO),carbon dioxide (CO₂) and mixtures thereof, and the combined gas streamis passed in contact with a catalyst comprising a supported metal oxideof a metal selected from the group consisting of titanium (Ti),zirconium (Zr), molybdenum (Mo), rhenium (Re), vanadium (V), chromium(Cr), tungsten (W), manganese (Mn), niobium (Nb), tantalum (Ta) andmixtures thereof to convert said carbon oxide and hydrogen sulfidemixture to methyl mercaptans, (primarily methanethiol (CH₃ SH) and asmall amount of dimethyl sulfide (CH₃ SCH₃)). The methyl mercaptans canbe used as a starting material for making additional products andpreferably are then passed in contact with a catalyst comprising certainsupported metal oxides or certain bulk metal oxides in the presence ofan oxidizing agent and for a time sufficient to convert at least aportion of the methyl mercaptans to formaldehyde (CH₂ O) and sulfurdioxide (SO₂). The carbon oxide used to react with the hydrogen sulfidepreferably is recovered as a by-product of the partial oxidation of themethane into formaldehyde over a catalyst that promotes the partialoxidation of methane to formaldehyde, such as a silica supported metaloxide catalyst of a metal selected from the group consisting of vanadium(V), niobium (Nb), molybdenum (Mo), chromium (Cr), rhenium (Re),tungsten (W), manganese (Mn), titanium (Ti), zirconium (Zr), tantalum(Ta) and mixtures thereof

2. Description of Related Art

Natural gas recovered from geological formations often contains hydrogensulfide as an undesired impurity in concentrations of 10-30%. Thehydrogen sulfide is typically separated from the methane and often isconverted to elemental sulfur via the Claus Process. In the ClausProcess, a first portion of the separated hydrogen sulfide is converted(oxidized) to sulfur dioxide (SO₂) and the remaining portion of thehydrogen sulfide is reacted with the sulfur dioxide in the presence of asuitable catalyst to produce water and elemental sulfur. The so-producedsulfur represents a low value-added, commodity product; while thede-sulfurized methane typically is distributed for industrial andpersonal uses, such as for home heating.

There have been several investigations aimed at developing a process fordirectly converting (partially oxidizing) methane directly to thevaluable chemical commodity formaldehyde. In one approach, the methaneis partially oxidized over a silica-based catalyst containing a surfacelayer of vanadium (V), molybdenum (Mo) and the like. See Sun et al.,Methane and Alkane Conversion Chemistry, pp. 219-226 (1995); Herman etal., Catalysis Today, 37:1-14 (1997) and Sun et al., J. of Catalysis,165, 91-101 (1997). Unfortunately, the potential for commercializing thecurrent methane partial oxidation catalyst technology is hindered by lowformaldehyde yields. A sizable fraction of the methane that is consumedis converted to carbon oxide by-products. As a result, the processremains commercially uneconomical relative to the standard indirectmethane-to-formaldehyde conversion process, i.e., steam reformingmethane to carbon monoxide and hydrogen; water gas shifting to enhancethe hydrogen to carbon monoxide ratio; hydrogenating the carbon monoxideto methanol and partially oxidizing the methanol to formaldehyde.

Ratcliffe et al., U.S. Pat. No. 4,570,020 describes a catalytic processfor producing methanthiol (CH₃ SH) from a gaseous feed comprising amixture of carbon dioxide (CO) and hydrogen sulfide (H₂ S). According tothe patent, the gaseous mixture is contacted, at a temperature of atleast about 225° C. with a catalyst comprising a metal oxide of a metalselected from the group consisting of vanadium (V), niobium (Nb), andtantalum (Ta) and mixtures thereof supported as an oxide layer ontitania. The methanethiol is disclosed as being useful as an odorant ortracer for natural gas and as a raw material for making methionine,fungicides and jet fuel additives.

The art has also identified methyl mercaptans, such as methanethiol (CH₃SH) and dimethyl sulfide (CH₃ SCH₃), as hazardous pollutants, and hassuggested a variety of ways for their destruction. Noncatalytic gasphase oxidation of such reduced sulfur compounds has been shown toproduce primarily sulfur oxide and carbon oxide products. A. Turk etal., Envir. Sci. Technol 23:1242-1245 (1989). Investigators haveobserved that oxidation in the presence of single crystal metal surfaces(Mo, Ni, Fe, Cu) results in the formation of methane and ethane,nonselective decomposition to atomic carbon, gaseous hydrogen and thedeposition of atomic sulfur on the metal surface via a stoichiometricreaction (See Wiegand et al., Surface Science, 279(1992):105-112).Oxidation of higher mercaptans, e.g., propanethiol on oxygen-coveredsingle crystal metal surfaces (Rh), produced acetone via astoichiometric reaction at low selectivity and accompanied by sulfurdeposition on the metal surface (See Bol et al., J. Am. Chem. Soc.,117(1995):5351-5258). The deposition of sulfur on the metal surfaceobviously precludes continuous operation.

The art also has disclosed using catalysts comprising a two-dimensionalmetal oxide overlayer on titania and silica supports, e.g., vanadia ontitania, for catalytically reducing NO_(x) by ammonia to N₂ and H₂ O inthe presence of sulfur oxides. Bosch et al., Catal. Today 2:369 et seq.(1988). Thus, such catalysts are known to be resistant to poisoning bysulfur oxides. It also is known that such catalysts, as well as certainbulk metal oxides catalysts, can be used to oxidize methanol toformaldehyde selectively. Busca etal, J. Phys. Chem. 91:5263 et seq.(1987).

Applicant recently discovered that supported metal oxide catalysts, suchas vanadia on titania, can be used to oxidize methyl mercaptans, such asmethanethiol (CH₃ SH) and dimethyl sulfide (CH₃ SCH₃), selectively toformaldehyde in a continuous, heterogenous catalytic process withoutbeing poisoned by the reduced sulfur. On the basis of that discovery,applicant now has envisioned the present process as a way of convertinga greater portion of the methane in a sour natural gas stream toformaldehyde so as to make the direct methane-to-formaldehyde conversionprocess more commercially attractive.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic drawing of a preferred embodiment of the processof the present invention.

FIG. 2 illustrates the distribution of products produced by oxidizingmethanethiol over a vanadia on titania catalyst containing about 1% byweight vanadia over the temperature range of 150° to 450° C. Maximumselectivity for the conversion of methanethiol to formaldehyde wasobserved at a temperature of about 400° C. Starting at about 300° C.,there was a significant conversion of methanethiol to formaldehyde.

BRIEF DESCRIPTION OF THE INVENTION

The present invention provides a process for facilitating the productionof formaldehyde directly from a gas stream containing methane (CH₄) andis particularly useful for processing a sour natural gas streamcontaining a mixture of methane (CH₄) and hydrogen sulfide (H₂ S) intoformaldehyde and possibly other chemical products.

More particularly, this invention provides a method wherein methane ispassed in contact with a methane-to-formaldehyde partial oxidationcatalyst, such as for example a supported metal oxide of vanadia onsilica, in the presence of an oxidizing agent and for a time sufficientto convert at least a portion of the methane to formaldehyde andby-product carbon oxides (CO and CO₂). The formaldehyde is recoveredseparate from the carbon oxides. The carbon oxides then are combinedwith hydrogen sulfide and the combined gas stream is passed in contactwith a catalyst comprising a supported metal oxide of a metal selectedfrom the group consisting of titanium (Ti), zirconium (Zr), molybdenum(Mo), rhenium (Re), vanadium (V), chromium (Cr), tungsten (W), manganese(Mn), niobium (Nb), tantalum (Ta) and mixtures thereof to convert saidcarbon oxides and hydrogen sulfide to methyl mercaptans, (methanethiol(CH₃ SH) and dimethyl sulfide (CH₃ SCH₃)), which can be used as a rawmaterial for the synthesis of other chemical products.

Preferably, the methyl mercaptans are passed thereafter in contact witha catalyst comprising certain supported metal oxides or certain bulkmetal oxides in the presence of an oxidizing agent and for a timesufficient to convert at least a portion of the methyl mercaptans toformaldehyde (CH₂ O) and sulfur dioxide (SO₂) and the formaldehyde isrecovered separate from the sulfur dioxide. The sulfur dioxide canthereafter be converted to sulfuric acid using known techniques.

In a preferred embodiment, a sour natural gas stream containing amixture of methane (CH₄) and hydrogen sulfide (H₂ S) first is treated toseparate methane from the hydrogen sulfide, the methane is used in thedirect methane-to-formaldehyde conversion step of the process, while atleast a portion of the so-separated hydrogen sulfide is reacted with theby-product carbon oxides recovered from the direct conversion step toproduce the methyl mercaptan.

DETAILED DESCRIPTION OF THE INVENTION

As noted, in a preferred embodiment the present invention is directed toa method for facilitating (e.g., increasing) the production offormaldehyde from a methane-containing gas stream, and especially from asour natural gas stream containing a mixture of methane (CH₄) andhydrogen sulfide (H₂ S). A key feature of the process is the selectiveconversion of carbon oxide, wherein the carbon oxide is selected fromcarbon monoxide (CO), carbon dioxide (CO₂) and mixtures thereof, such asmay be produced from the direct partial oxidation of methane toformaldehyde or recovered from natural gas, first to methyl mercaptans,such as methanethiol (CH₃ S) (following combination of the carbon oxidewith hydrogen sulfide (H₂ S), such as may be recovered from a sournatural gas stream), and then to an additional chemical product,especially formaldehyde and sulfur dioxide. The so-produced formaldehydeis recovered and can be combined with the formaldehyde produced by thedirect, partial oxidation of methane to formaldehyde, which can proceedusing known catalytic processes, to result is an overall yield increasein formaldehyde production from the sour natural gas (methane).

Thus, in one important aspect, the process of the present inventioninvolves flowing a gaseous stream containing carbon oxides and hydrogensulfide (H₂ S) in contact with a catalyst comprising a metal oxide of ametal selected from the group consisting of vanadium (V), niobium (Nb),molybdenum (Mo), chromium (Cr), rhenium (Re), titanium (Ti), tungsten(W), manganese (Mn), tantalum, zirconium (Zr), and mixtures thereofsupported, for example, on titania, silica, zirconia, alumina, ceria,magnesia, niobia, lanthanum oxide, tin oxide or their mixture,preferably on titania, zirconia, ceria, niobia, tin oxide or theirmixture, more preferably on titania. As a general rule, the support andthe supported metal should not be the same. The gaseous mixture iscontacted with the metal oxide catalyst at a temperature of at leastabout 225° C. for a time sufficient to convert at least a portion of thecarbon oxide and H₂ S to methyl mercaptan, primarily methanethiol. Thegaseous stream containing the methyl mercaptan can then be used as afeedstock for applications where methyl mercaptans are used. Forexample, in a preferred embodiment of this invention, the methylmercaptan can thereafter be contacted with a supported metal oxide orbulk metal oxide catalyst, under oxidizing conditions and for a timesufficient to convert at least a portion of the methyl mercaptan toformaldehyde and sulfur dioxide. The formaldehyde then can be recoveredas a product separate from the gas stream.

In carrying out this aspect of the process of the present invention, themetal oxide overlayer of the supported metal oxide used in the processof converting methyl mercaptan to formaldehyde is typically based on ametal selected from the group consisting of titanium (Ti), zirconium(Zr), molybdenum (Mo), rhenium (Re), vanadium (V), chromium (Cr),tungsten (W), manganese (Mn), niobium (Nb), tantalum (Ta) and mixturesthereof and the support generally is selected from titania, silica,zirconia, alumina, ceria, magnesia, niobia, lanthanum oxide, tin oxideand mixtures thereof. Metal oxide catalysts made with a support ofsilica, alumina or magnesia, and especially an alumina support, performmore effectively when the reaction between the carbon oxide and hydrogensulfide is conducted in the added presence of hydrogen. Generally, acatalyst having a support of titania, zirconia, ceria, niobia or theirmixture is preferred. As a general rule, especially when the reactionbetween carbon oxide and hydrogen sulfide is conducted in the absence ofhydrogen, titanium (Ti), zirconium (Zr), niobium (Nb), tantalum (Ta) andtungsten (W) should not be used as the sole catalytic species with asilica, alumina or magnesia support, nor should the support and thesupported metal be the same. In an alternative embodiment of the presentinvention, the optional step of the process, involving the conversion ofmethyl mercaptan to formaldehyde, also can be carried out using a bulkmetal oxide catalyst wherein the bulk metal oxides, and especially bulkmixed metal oxides, are based on molybdates (Mo), chromates (Cr),vanadates (V), rhenates (Re), titanates (Ti), niobates (Nb), tungstates(W) and mixtures thereof Mulk metal oxide catalysts based on molybdenum,chromium and vanadium are preferred.

The supported metal oxide and bulk metal oxide catalyst compositionsuseful for practicing the present invention are known in the prior art.

In a preferred embodiment, a supported metal oxide catalyst is used inboth the reaction of carbon oxides with hydrogen sulfide and in the stepof converting methyl mercaptan to formaldehyde and is preferably basedon vanadium (V) oxide supported on titania. For the supported metaloxide catalyst used in the mercaptan to formaldehyde step, the vanadiumoxide may preferably be used in mixture with an oxide of one ofmolybdenum (Mo), tungsten (W), chromium (Cr), rhenium (Re), niobium(Nb), tantalum (Ta) and manganese (Mn), supported on titania or silica.In the case of a vanadia on silica catalyst, an adjuvant selected fromthe group consisting of titanium, zirconium, cerium, tin, niobium andtantalum, should be present to enhance catalytic activity. As notedabove, vanadia on titania is particularly preferred as a metal oxidesupported catalyst in both of these steps according to this aspect ofthe inventive process.

In accordance with the present invention, and with reference to FIG. 1,one embodiment of the present invention is described. As noted, onesuitable source for both the methane and hydrogen sulfide (and alsocarbon oxides) used in carrying out the process of the invention can bea sour natural gas. In the FIG. 1 process, this sour natural gas stream10 is delivered to a separation zone 200 from which separate streams ofhydrogen sulfide 20 and methane 40 are produced. Methods for separatingthe hydrogen sulfide (and carbon oxides) from the sour natural gascontaining methane and hydrogen sulfide are well known and have longbeen practiced in the art, including fractional condensation, absorptionand adsorption techniques. Thus, such methods need no furtherdescription. Any of such widely available methods can be used to producethe separate gas streams of methane 40 and hydrogen sulfide 20, used topractice this embodiment of the present invention.

The so-separated methane 40 then is contacted in the presence of anoxidizing agent, such as molecular oxygen, introduced through stream 30,with a suitable methane partial oxidation catalyst, such as a supportedmetal oxide catalyst of vanadia on silica, in reaction zone 210 topartially oxidize the methane directly to formaldehyde. Reactor 210preferably should be operated in a way to ensure that there is noresidual oxidizing agent in stream 50. Other methane partial oxidationcatalysts suitable for conducting this partial oxidation are known tothose skilled in the art and include, for example, a high surface areasilica, molybdate on silica, rhenia on silica, tungsta on silica, niobiaon silica, zirconia on silica and mixtures thereof A more comprehensivelisting of methane partial oxidation catalysts suitable for the directconversion of methane to formaldehyde is found in Herman et al.,Catalysis Today, 37:1-14 (1997), which is incorporated herein byreference. Silica-based catalysts are presently preferred as methanepartial oxidation catalysts for the methane to formaldehyde reaction.

Since the present invention provides a productive use for the carbonoxide co-products produced during this direct, partial oxidation ofmethane to formaldehyde, the relatively low selectivity oftenencountered in the methane to formaldehyde conversion with these priorart methane partial oxidation catalysts is less of a concern. Rather,the co-product carbon oxides are advantageously used to produce a methylmercaptan stream in tandem with the methane partial oxidation, whichmercaptan can be used for further chemical synthesis, particularly forthe preparation of additional formaldehyde.

In this regard, the formaldehyde-containing product gas stream in line50, also containing a sizeable quantity of carbon oxides and unreactedmethane, is passed to a separation zone 220 where a formaldehyde productand water is recovered in stream 60 separate from a carbonoxide-enriched stream in line 70, such as by fractional condensation orabsorption processes. Again, such methods for separating the carbonoxides from formaldehyde and water are well known and need no furtherdescription. Any of the widely available methods can be used to producethe separate streams of formaldehyde 60 and carbon oxide 70. Preferably,the gas stream also is processed to remove at least some of theunreacted methane, which can be recycled (not shown) to the partialoxidation reactor, the remainder comprises an unreactive diluent in thesubsequent reaction zone 230.

In accordance with this embodiment of the invention, the carbon oxidegas stream 70 is combined with hydrogen sulfide (H₂ S) gas provided inline 20 for reaction in catalytic reaction zone 230. In this embodiment,the hydrogen sulfide is produced by separating it from a sour naturalgas. In reactor 230, the combined H₂ S and carbon oxide reactants arepassed in contact, at a temperature of at least 225° C., with a catalystcomprising a metal oxide of a metal selected from the group consistingof vanadium (V), niobium (Nb), molybdenum (Mo), chromium (Cr), rhenium(Re), titanium (Ti), tungsten (W), manganese (Mn), tantalum, zirconium(Zr), and mixtures thereof supported on titania, silica, zirconia,alumina, ceria, magnesia, niobia, lanthanum oxide, tin oxide or theirmixture. Preferably, the cartalyst comprises a titania supported metaloxide of a metal selected from the group consisting of vanadium (V),niobium (Nb), molybdenum (Mo), chromium (Cr), rhenium (Re), titanium(Ti), tungsten (W), manganese (Mn), tantalum (Ta), and mixtures thereofAlternatively, the support is preferably selected from zirconia, ceria,tin oxide, niobia or their mixture with each other and with titania. Ineach case, the support and the supported metal generally should not bethe same. The contacting is conducted for a time sufficient to convertthe mixture of carbon oxides and hydrogen sulfide to methyl mercaptans,(e.g., methanethiol (CH₃ SH) and dimethyl sulfide (CH₃ SCH₃)).Especially when using an alumina support, the contacting also should beconducted in the presence of hydrogen.

As described in U.S. Pat. No. 4,570,020, the onset of methanethiolproduction from a mixed gas feed of CO and H₂ S starts to occur at atemperature of about 225° C., is more pronounced at about 250° C., andreaches a peak (under the particular reaction conditions and feedmixture reported in that patent) or optimum at a temperature of about325° C. However, methanethiol tends to encounter thermal instability at350° C. Therefore, if the reaction temperature approaches and/or exceedsthis temperature, the prior art suggests that one take provisions toquench the so-formed methanethiol rapidly down to a temperature belowabout 350° C. in order to prevent thermal decomposition thereof.

In general, the ratio of H₂ S to CO in the gaseous feed used in thisstep of the process of this invention will typically range from about1/4 to 40/1 and more preferably from about 1/2 to 4/1 on a mole basis.As reported in U.S. Pat. No. 4,570,020, it was found experimentally thata 1/1 ratio appears to be the optimum for maximum conversion of thefeed. The reaction temperature will generally range from about 225° to450° C., preferably 250° to 400° C. and still more preferably from about300° to 400° C.

However, as previously stated, the prior art suggests that exceeding areaction temperature of 350° C. may necessitate a quenching of themethyl mercaptan product recovered from this step in order to reduce oravoid its thermal decomposition. With regard to the space velocity ofthe feed, it is understood that longer contact times will result in agreater amount of desired product. However, this may be offset bydecomposition of methyl mercaptan product in contact with the catalyst.In general, the space velocity will be maintained below about 4800V/V/hr.

The reactor 230 is preferably operated at a superatmospheric pressure.An operating pressure up to 50 atmospheres is expected to be suitable,e.g., a pressure of 30-50 atmospheres. Because the reaction of CO and H₂S is preferably conducted in the gas phase, the total reactor pressureshould not result in a partial pressure of H₂ S that would cause the H₂S to condense. Accordingly, unless it is desired in a specificcircumstance that the reaction occur in the presence of both gaseous andliquid phases, the reactor conditions should be maintained to avoid H₂ Scondensation.

The methyl mercaptan stream can be recovered or can be used as a rawmaterial for the synthesis of a variety of chemical products. In thepresent embodiment, the methyl mercaptan-containing stream 80 exitingreaction zone 230, following removal of any unreacted hydrogen sulfide,by-product hydrogen, COS, and possibly unreacted carbon oxides (and anymethane not previously removed), thereafter contacts, in the gas phase,a supported metal oxide or a bulk metal oxide catalyst in reaction zone240. The removal of the noted constituents is accomplished in separationzone 235. As will be recognized by those skilled in the art, a varietyof separation processes can be used to remove any undesired constituentsfrom the desired mercaptan product, including absorption, adsorption andfractional condensation techniques, prior to reaction zone 240. Themercaptan stream 95 then contacts the supported metal oxide or bulkmetal oxide catalyst under oxidizing conditions at a temperature in therange of 200° to 700° C., preferably in the range of 300° to 600° C.,and most often in the range of 325° to 500° C. The operating pressurefor the catalytic reactor 240 is not critical. Operation at atmosphericpressure has been found suitable.

Air or enriched oxygen generally is added via stream 90 to the mercaptanproduct stream to establish oxidizing conditions in the reaction zone240. The selective oxidation of the methyl mercaptan producesformaldehyde and sulfur dioxide, which exits reaction zone 250 in stream100. As noted, the oxidizing agent used in the selective methylmercaptan oxidation can usually be oxygen or air. The contacting of themethyl mercaptan with the supported metal oxide catalyst or bulk metaloxide catalyst under an oxidizing atmosphere, e.g., in the presence ofoxygen, and at an appropriate temperature, causes a selective conversionof the methyl mercaptan to formaldehyde. As with reactor 210, reactor250 preferably should be operated in a way to ensure that there is noresidual oxidizing agent in stream 100.

Formaldehyde is the intended product of this preferred embodiment of thepresent process and it can be recovered from the gaseous reactionproducts 50 and 100 in separation zones 220 and 250 using anyone of anumber of ways known to those skilled in the art.

In particular, as will be recognized by those skilled in the art, thegases 50 and 100 leaving the reaction zones 210 and 240, respectively,may contain unreacted starting products, including any inert gases thatmay have been added, as well as formaldehyde and water. The principalby-products formed in the partial oxidation of methane are carbonoxides; while the principal by-products formed in the partial oxidationof methyl mercaptans include carbon oxides (mainly carbon monoxide,which may be accompanied by a small amount of carbon dioxide), andsulfur dioxide. COS will likely be a sizable by-product of the H₂ S andCO reaction; but only a minor product of the partial oxidation of methylmercaptan.

The formaldehyde-containing reaction mixtures leaving the partialoxidation reaction zones 210 and 240 are generally subject to furtherprocessing in separation zones 220 and 250, respectively, in aconventional manner. For example, the formaldehyde product can beseparated in a washer, or by indirect cooling, or also by fractionalcooling. For example, the washing can be performed with water, in whichcase a multi-stage washer can be used. Aqueous formaldehyde solutions 60and 120 can be obtained in this manner. From a combined solution 130,commercial formaldehyde solutions can be prepared by distillation forsubsequent technical use. The formaldehyde also can be condensed out ofthe reaction gas together with the water that has formed. In thismanner, concentrated formaldehyde solutions in common commercial formmay be obtained. Other ways for isolating the formaldehyde product fromthe product gases will be apparent to those skilled in this art. Sulfurdioxide formed in reaction zone 240 also can be removed in separationzone 250, or alternatively can be removed from stream 110 beforerecycling. Methods for removing SO₂ from these streams will berecognized by those skilled in the art, again absorption, adsorption andfractional condensations techniques should be suitable. The SO₂ can beoxidized to sulfuric acid. Residual gas 110 from separation zone 250,possibly containing carbon oxides may be recycled to the methylmercaptan production step in reaction zone 230.

For obtaining higher yields and selectivities in the conversion ofmethyl mercaptan to formaldehyde in reaction zone 240, it may bedesirable to conduct the reaction such that only a partial reactiontakes place in a single pass through the reactor. For example, thepressure, temperature, composition of the starting gas mixture, theamount of catalyst and/or the rate of flow can be varied to cause apartial conversion of the mercaptan feed. In this case the separationzone, 250 needs to accommodate the separation of the formaldehydeproduct, carbon oxides, SO₂ and the like from the unreacted mercaptan.Separation of carbon oxides from the SO₂ also would permit recycle ofthe carbon oxides. The Citrex process for preferentially absorbing SO₂is one option. The reactor effluent gas remaining after separation ofthe formaldehyde and the other constituents from the mercaptans couldthen be recycled (not shown) into the reactor 240. It is desirable toadd to this gas via line 80 the amount of methyl mercaptan that has beenconsumed. In this manner, a continuous circulation can be achieved. Ifthe gas is recirculated in this manner, the inert gases and by-productgases, especially carbon oxides, may concentrate in the recycled gas.Any excessive accumulation of these gases can be prevented by acontinuous or discontinuous side-stream removal, which, as noted above,may be advantageously recycled for reaction with hydrogen sulfide inaccordance with this aspect of the process. It is also desirable toreplace the removed exhaust gas with an equal amount of fresh gas. Theprocess may also benefit from the use of one or more purge streams (notshown) to maintain a favorable mass balance, as known to those skilledin the art.

The catalysts useful in promoting the reaction of carbon monoxide andhydrogen sulfide to methyl mercaptans comprise a support of titania,zirconia, ceria, tin oxide, niobia or their mixture whose surface hasbeen modified with an oxide of a metal selected from vanadium (V),niobium (Nb), molybdenum (Mo), chromium (Cr), rhenium (Re), titanium(Ti), tungsten (W), manganese (Mn), tantalum (Ta) and mixtures thereof.That is, the surface of the titania, zirconia, ceria, niobia or theirmixture has been modified by an oxide layer of vanadium (V), niobium(Nb), molybdenum (Mo), chromium (Cr), rhenium (Re), titanium (Ti),tungsten (W), manganese (Mn), tantalum (Ta) or a mixture thereof in anamount such that the catalyst exhibits properties different from titaniawhose surface has not been modified and different from bulk oxides ofvanadium, niobium, molybdenum, chromium, rhenium, tungsten, tantalum ora mixture thereof As a general rule, the support and the supported metalshould not be the same. Consequently, the metal oxide loading on thesupport should be sufficient to modify the titania, zirconia, ceria,niobia, tin oxide or their mixture surface, but preferably is not enoughto result in a catalyst exhibiting properties of the bulk oxides ofvanadium, niobium, molybdenum, chromium, rhenium, titanium, tantalum ora mixture thereof Thus, at least a portion of, and preferably at leastabout 30 wt. % of, the metal oxide will be in a non-crystalline form.This will be accomplished if the metal oxide loading on the metal oxidesupport broadly ranges between about 0.5 to 30 wt. % of the totalcatalyst weight.

The metal oxide of the supported metal oxide catalyst in the reactionzone 240 also is accommodated in the support primarily as atwo-dimensional metal oxide overlayer (possibly a monolayer), with theoxide having a non-crystalline form. Supported metal oxide catalystsuseful in this reaction zone generally comprise a metal oxide substrate,such as titania, silica, zirconia, alumina, niobia, ceria, magnesia,lanthanum oxide, tin oxide and mixtures thereof, whose surface has beenmodified with a layer of an oxide of a metal or a mixture of metals asidentified above (e.g., preferably vanadium, and mixtures containingvanadium) in an amount such that the catalyst exhibits propertiesdifferent from the metal oxide substrate whose surface has not beenmodified. These catalysts also behave differently from bulk metal oxidesof the metal oxide overlayer (e.g., bulk oxides of vanadium, and itsmixtures). Consequently, in this embodiment of the invention, the metaloxide loading on the metal oxide support or substrate, e.g., titania,must be sufficient to modify the metal oxide surface, but not enough toresult in a catalyst exhibiting properties of the bulk oxides of themetal oxide overlayer, e.g., a bulk oxide of vanadia. Thus, at least aportion of and preferably at least about 25 wt % of the metal oxidecoating will be in a non-crystalline form. This will be accomplished ifthe metal oxide loading on the metal oxide support or substrate broadlyranges between about 0.5 to 35 wt % of the total catalyst weight.

A preferred metal oxide support for use in both steps 230 and 240 ofthis embodiment of the process is titania (titanium dioxide) which canbe employed in the anatase or rutile form. For example at least about 25wt % (and generally from about 50 to about 100 wt %) of the titaniumdioxide (TiO₂) can be in the anatase form. As recognized by thoseskilled in the catalytic art, the titania support material shouldpreferably be judiciously evaluated since certain grades may haveimpurities that interfere with the catalytic activity. Normally, withrecognition of the previous caveat, the titanium dioxide may be preparedby any conventional technique. The titanium dioxide used in thecatalysts of this aspect of the invention may be composed ofsubstantially porous particles of a diameter of from about 0.4 to about0.7 micron and preferably has a specific surface area of at least about1 m² /g.

The metal oxide supported catalysts used in any of the steps of theprocess of this invention may be prepared by impregnation techniqueswell-known in the art, such as incipient wetness, grafting, equilibriumadsorption, vapor deposition, thermal spreading, etc. When using anincipient wetness impregnation technique, an aqueous or non-aqueoussolution containing a metal oxide precursor compound(s) is contactedwith the metal oxide support or substrate material, e.g., silica ortitania, for a time sufficient to deposit a metal oxide precursormaterial onto the support such as by selective adsorption oralternatively, excess solvent may be evaporated leaving behind theprecursor compound or salt. If an incipient wetness impregnationtechnique is used to prepare a catalyst of this invention, the metaloxide precursor (e.g., salt) solution used may be aqueous or organic,the only requirement being that an adequate amount of a precursorcompound for the selected metal oxide be soluble in the solvent used inpreparing this solution. Other impregnation techniques, such as vapordeposition and thermal spreading, do not require use of a solvent asdoes incipient wetness, and may be desirable in some circumstances toavoid the problem of volatile organic carbon (VOC) emissions.

For example, one way to disperse vanadium oxide, tungsten oxide or acombination of the two oxides onto a titania metal oxide support orsubstrate is to impregnate titania spheres or powder (spheres or powderare used as representative examples of shapes of titania) with asolution containing a vanadium or a tungsten compound. When impregnatinga substrate with both oxides, the tungsten and vanadium are introducedin a stepwise manner, tungsten first, followed by vanadium, withappropriate intermediate drying and calcining steps. Each solution maybe an aqueous solution, one using an organic solvent or a mixture of thetwo. Generally, an aqueous solution is preferred. Criteria used tochoose the vanadium and tungsten compounds include whether the compoundsare soluble in the desired solvent and whether the compounds decomposeat an acceptable rate at a high, calcination temperature to give theappropriate metal oxide. Illustrative of suitable compounds of vanadiumand tungsten are the halides of vanadium and tungsten, oxyacids, oxyacidsalts and oxysalts of vanadium and tungsten. Specific examples aretungsten dibromide, tungsten pentabromide, tungsten tetrachloride,tungsten dioxydichloride, tungstic acid, ammonium meta-tungstate,vanadium tribromide, vanadium dichloride, vanadium trichloride, vanadiumoxychloride, vanadium oxydichloride, vanadic acid, vanadyl sulfate,vanadium alkoxides, vanadium oxalate (which may be formed in situ byreaction of V₂ O₅ and an aqueous solution of oxalic acid), and ammoniummeta-vanadate. Suitable metal oxide precursor compounds for the othermetal species suitable for making the supported metal oxide catalysts ofthis invention are well recognized by those skilled in the catalysisart.

The impregnation of the metal oxide support or substrate, e.g., titania,silica or alumnia support spheres or powder, with the metal oxideprecursor compound solution may be carried out, as noted above, in wayswell known in the art using either wet or dry impregnation techniques.One convenient method is to place the metal oxide support or substrate,e.g., titania, silica or alumina particles, into a rotary evaporatorwhich is equipped with a steam jacket. An impregnating solution of aprecursor compound which contains an amount of the desired metal to beincluded in the finished catalyst (as the metal) is added to the supportparticles and the mixture is cold rolled (no steam) for a time fromabout 10 to 60 minutes sufficient to impregnate the support with theprecursor compound solution. Next, steam is introduced and the solventis evaporated from the impregnated solution. This usually takes fromabout 1 to about 4 hours. The impregnated support will normally be driedat temperatures ranging from about 50°-300° C. to remove excess solvent.

Water soluble precursor compounds are generally preferred for industrialapplications because of the environmental concern about VOC emissions.Nonetheless, when using an organic solvent initial heating may be donein a nitrogen atmosphere to remove any flammable solvent. Finally, thesupport particles are removed from the rotary evaporator and calcined ina suitable oxidizing atmosphere such as air, oxygen, etc. at atemperature of about 150° to 800° C., and more usually from 400°-600°C., preferably for about 1 to about 3 hours, sufficient to decompose theprecursor compound to the corresponding metal oxide. In other cases, asrecognized by those skilled in the art, calcining conditions need to beadjusted to avoid undesirably reducing surface area.

Because some precursor compounds are air/moisture sensitive, they arecommonly prepared under a nitrogen atmosphere as is recognized by thoseskilled in this art. The time required to calcine the composite will, ofcourse, depend on the temperature and in general will range from about0.5-7 hours. Calcination at 450° C. for about 2 hours has proven to besuitable for 1% vanadia on titania catalyst. The precise time andtemperature for calcination depends on the particular metal oxideoverlayer and should be selected to avoid adversely affecting the metaloxide support, e.g., in the case of a titania metal oxide support, toavoid substantial crystal phase transformation of the anatase intoanother crystalline form, e.g., rutile, and degradation of extendedsurface areas. Selection of a suitable combination of time andtemperature is a matter of routine testing to those skilled in the art.

Reducing atmospheres may also be used to decompose the transition metaloxide precursors. To avoid potential safety concerns, the resultingcomposite should be calcined to convert the reduced metal component tothe oxide form. If the support is to be provided with an overlayer of acombination of metal oxides, e.g., if an overlayer containing bothvanadium and tungsten oxide is desired, then the metal oxide precursorcompounds may be impregnated on the metal oxide support simultaneously,but preferably are impregnated sequentially as previously noted.

The metal oxide supported catalysts used in the process of thisinvention will generally have surface metal oxide loadings of from about0.5 to 35 wt. % metal oxide based on the total active catalystcomposition, preferably from about 1 to 20 wt. %, more usually fromabout 1-15 wt. %, and most preferably 1-10 wt. % based on the totalactive catalyst composition.

While titania, silica, zirconia, alumina, niobia, ceria, magnesia,lanthanum oxide and tin oxide are conveniently referred to as supportsor substrates in the description of the preferred embodiment of thepresent invention, based to a large degree on the way the catalysts areprepared, it should be noted that they provide important roles as activecatalytic components in the supported metal oxide catalyst. Combinationsupports may also be advantageous for use in catalysts suitable forpracticing the methyl mercaptan to formaldehyde conversion step of thepreferred embodiment of the invention. For example, substratesconstituting a mixture of titania and zirconia or titania and silica canbe used.

Further details on the preparation and structure of metal oxidesupported catalysts useful in the practice of the present invention canbe found in Jehng et al., Applied Catalysis A, 83, (1992) 179-200; Kimand Wachs, Journal of Catalysis, 142, 166-171; Jehng and Wachs,Catalysis Today, 16, (1993) 417-426; Kim and Wachs, Journal ofCatalysis, 141, (1993) 419-429; Deo et al., Applied Catalysis A, 91,(1992) 27-42; Deo and Wachs, Journal of Catalysis, 146, (1994) 323-334;Deo and Wachs, Journal of Catalysis, 146, (1994) 335-345; Jehng et al.,J. Chem. Soc. Faraday Trans., 91(5), (1995) 953-961; Kim et al., Journalof Catalysis, 146, (1994) 268-277; Banares et al., Journal of Catalysis,150, (1994) 407-420 Jehng and Wachs, Catalyst Letters, 13, (1992) 9-20;Sun et al., Methane and Alkane Conversion Chemistry, pp.219-226 (1995);Herman et al., Catalysis Today, 37:1-14 (1997) and Sun et al., J. ofCatalysis, 165, 91-101 (1997), the disclosure of which are incorporatedherein by reference.

Preferred supported metal oxide catalysts for the step of partialoxidation of methyl mercaptans to formaldehyde are those which are knownto be suitable for converting methanol to formaldehyde. Particularlypreferred are supported metal oxide catalysts comprising a vanadiaoverlayer on a titania support.

It often is desired that the metal oxide, such as titania, silica,zirconia, alumina niobia, magnesia, ceria, lanthanum oxide, tin oxide,and their mixtures, used as a catalyst support component in accordancewith the present invention have a surface area in the range of about 5to about 150 m² /g and higher. These materials may be used in anyconfiguration, shape or size which exposes their surface and any metaloxide layer dispersed thereon to the gaseous stream passed in contacttherewith. For example, these oxide supports, such as titania canconveniently be employed in a particulate form or deposited (before orafter impregnation with the metal oxide overlayer) on a monolithiccarrier or onto ceramic rings or pellets. As particles, the support,such as titania, can be formed in the shape of pills, pellets, granules,rings, spheres and the like. Use of free particulates might be desirablewhen large catalyst volumes are needed or if the catalyst bed isoperated in a fluidized state. A monolithic form or deposition of theactive catalyst on an inert ceramic support might be preferred inapplications where catalyst movement is to be avoided because ofconcerns about catalyst attrition and dusting, and a possible increasein pressure drop across a particulate bed. In a preferred approach, ametal oxide supported catalyst, such as a vanadia on titania catalyst,may be deposited on a ceramic carrier such as silicon carbide, siliconnitride, carborundum steatite, alumina and the like, provided in theshape of rings or pellets. Typically, the active catalyst will beapplied to the inert ceramic support in an amount to provide 1 to 20% byweight of the supported catalyst.

As noted, the present invention also contemplates the use of bulk metaloxides as the catalyst for converting methyl mercaptan to formaldehydein the preferred embodiment. Such bulk metal oxide catalysts generallyconstitute molybdates (Mo), chromates (Cr), vanadates (V), rhenates(Re), titanates (Ti), niobates (Nb), tungstates (W) and mixtures thereofSuch metal oxides also contain a wide variety of other metal speciessuch as alkali metals (e.g., sodium (Na), lithium (Li), potassium (K)and cesium (Cs)), alklanine earth metals (e.g., calcium (Ca), barium(Ba), and magnesium (Mg)) and transition metals (e.g., copper (Cu),nickel (Ni), cobalt (Co), aluminum (Al), lead (Pb), bismuth (Bi), iron(Fe), zinc (Zn), cadmium (Cd), tellurium (Te), manganese (Mn)). Thoseskilled in the art recognize the wide variety of available bulk metaloxide catalysts. As a general rule, those bulk metal oxide catalystsknown to be suitable for converting methanol to formaldehyde also may besuitable for the methyl mercaptan to formaldehyde conversion of thepresent invention.

Methods for making bulk metal oxide catalysts used in the presentinvention also are well known to those skilled in the art. Inparticular, the active catalyst can be prepared by physically blendingthe metal oxides, by coprecipitation from aqueous solutions containingsoluble compounds of the catalyst components in the desired molar ratioor by any other technique which provides an intimate mixture of themetal oxide constituents. For example, an aqueous solution of awater-soluble molybdenum compound (ammonium heptamolybdate) is mixedwith a water-soluble iron compound (ferric chloride) to causecoprecipitation of both molybdenum and iron, using procedures well knownto those skilled in the art. The coprecipitate is washed, to eliminatethe soluble salts formed during the coprecipitation reactions, filtered,dried and calcined to convert the metal constituents to their activeiron molybdate (oxide) form. Those skilled in the art recognize avariety of water soluble metal compounds that can be used to prepare theactive catalyst. Alternatively, oxides of the respective metals may beground together and calcined. Additional details on bulk metal oxidesand bulk metal oxide catalysis can be found in Arora et al., Journals ofCatalysis, 159, (1996) 1-13, which is incorporated herein by reference.

Those skilled in the art recognize that there exists a wide range ofcompounds, generally used in admixture, suitable for preparing bulkmetal oxide catalysts. The following is a representative, though notexhaustive, list of possible constituents: bulk vanadates such as PbV₂O₆, NaVO₃, Na₃ VO₄, BiVO₄ and other Bi--V--O family members, AlVO₄,FeVO₄, Mg₃ (VO₄)₂, Mg₂ V₂ O₇, CeVO₄, Zn₃ (VO₄)₂, CdV₂ O₇, Zn₂ V₂ O₇,VOPO₄ and other V--P--O family members, KVO₃, Pb₂ V₂ O₇, and TlVO₄ ;bulk molybdates such as PbMoO₄, CaMoO₄, Bi₂ Mo₂ O₉, Bi₃ (FeO₄)(MoO₄)₃and other Bi--Mo--O family members, Na₂ MoO₄, MnMoO₄, Gd₂ (MoO₄)₃,MgMoO₄, CuMoO₄, CoMoO₄, Fe₂ (MoO₄)₃, Te₂ MoO₇, CoMoO₄, Al₂ (MoO₄)₃, Cr₂(MoO₄)₃, and Na₂ Mo₂ O₇ ; bulk niobates such as YNbO₄, YbNbO₄, LiNbO₃,NaNbO₃, KNbO₃, AlNbO₄, K₈ Nb₆ O₁₉, BiNbO₄, and other Bi--Nb--O familymembers, SbNbO₄, NbOPO₄, CaNb₂ O₆, K₄ Nb₆ O₁₇, and KCa₂ Nb₃ O₁₀ ; bulktungstates such as Li₆ WO₆, FeWO₄, CoWO₄, MnWO₄, NiWO₄, CuWO₄, CaWO₄,Cs₂ WO₄, Na₂ WO₄, B_(a) WO₄, Fe₂ (WO₄)₃, Al₂ (WO₄)₃, SrWO₄, K₂ WO₄, Na₂W₂ O₇, Li₂ WO₄, CsLuW₂ O₈, BiWO₄, and other Bi--W--O family members;bulk chromates such as Na₂ CrO₄, Na₂ Cr₂ O₇, Na₂ Cr₃ O₁₀, Na₂ Cr₄ O₁₃,K₂ CrO₄, K₂ Cr₂ O₇, K₂ Cr₃ O₁₀, K₂ Cr₄ O₁₃, Fe₂ (CrO₄)₃, CaCrO₄, Cs₂CrO₄, BiCrO₄ and other Bi--Cr--O family members; bulk rhenates such asNaReO₄, Li₆ ReO₄, and Mg(ReO₄)₂ ; and bulk titanates such as Na₂ TiO₄,NaTiO₃, BaTiO₄, BaTiO₃, and other Ba--Ti--O family members.

To achieve high selectivity in the conversion of methyl mercaptan toformaldehyde it is important to maintain the flow rate of methylmercaptan per unit mass of catalyst in the range of 10⁻² to 10⁴ cubiccentimeters (STP) of methyl mercaptan per gram of active catalyst perminute (excluding inert ceramic components or other inert supportmaterial). Generally, higher reaction temperatures permit higher flowrates. Usually, the process can be operated at 0.1 to 100, cubiccentimeters (STP) of methyl mercaptan per gram of catalyst per minute.

As used herein, the term "selectively" is intended to embrace theconversion of at least 1% of the methyl mercaptan, preferably at least10% of the methyl mercaptan, more usually at least 50% of the methylmercaptan and most preferably at least 70% of the methyl mercaptan whichcontacts the catalyst to formaldehyde. Selectivity, as that term is usedherein, is determined by the percentage of formaldehyde in the mercaptanconversion products as a proportion of the carbon-containing mercaptanoxidation products.

The oxidation reaction of the methyl mercaptan to formaldehyde step isexothermic. As recognized by those skilled in the art a variety ofreactor designs may be employed to accommodate the necessary mass andheat transfer processes for effective operation on a continuous basis.The reaction may be conducted at atmosphere pressure, and above or belowatmospheric pressure.

EXAMPLES

To facilitate a more complete understanding of the invention, a numberof Examples are provided below. The scope of the invention, however, isnot limited to specific embodiments disclosed in these Examples, whichare for purposes of illustration only. These examples illustrate thataspect of the invention relating to the partial oxidation of methylmercaptans to formaldehyde.

Catalyst Preparation and Characterization

Supported metal oxide catalysts were prepared as follows:

PREPARATION EXAMPLE 1: VANADIA ON TITANIA

A vanadia on titania metal oxide supported catalyst was prepared inaccordance with the following procedure. The vanadia-titania catalystwas prepared by using TiO₂ (Degussa P25) as the support. The TiO₂support (˜10% rutile and ˜90% anatase) possessed a surface area of ˜55m² /g. It was calcined in air at 500° C. and cooled to room temperaturebefore impregnation with the vanadium oxide precursor. The vanadiumoxide overlayers on the TiO₂ support were prepared from vanadiumtriisopropoxide oxide (Alfa, 95-98% purity) by the incipient wetnessimpregnation method. The preparation was performed under a nitrogenenvironment and in nonaqueous solutions, since the alkoxide precursor isair and moisture sensitive. Solutions of known amounts of vanadiumtriisopropoxide oxide and propanol-2, corresponding to the incipientwetness impregnation volume and the final amount of vanadium required,were prepared in a glove box filled with nitrogen. The solutions of thevanadium precursor and propanol-2 were then thoroughly mixed with thetitania support and dried at room temperature in the glove box for 24hr. The impregnated samples were heated to 300° C. in flowing nitrogenand the final calcination was performed in O₂ (Linde, 99.9% pure) at500° C. for 15 hours. The catalyst was then pelletized, crushed andsieved to obtain catalyst particles sizes between 100 to 200 μm.

PREPARATION EXAMPLE 1 A: VANADIA ON TITANIA

Another vanadia on titania metal oxide supported catalyst was preparedusing the general procedure of Preparation Example 1 except that thefinal calcination was conducted at 450° C. for 2 hours.

PREPARATION EXAMPLE 2: MOLYBDENUM OXIDE ON TITANIA

An aqueous solution of ammonium heptamolybdate (NH₄)₆ Mo₇ O₂₄ ·4H₂ O)(Alfa) was deposited onto TiO₂ (Degussa P25) as the support (˜10% rutileand ˜90% anatase) by the incipient wetness technique. As in Example 1,the support was calcined in air at 500° C. and cooled to roomtemperature before impregnation with the molybdenum oxide precursor. Thesupport possessed a surface area of ˜55 m² /g. After impregnation, thewet samples were dried at room temperature for 16 hours, further driedat 110-120° C. for 16 hours and calcined at 450° C. for 12 hours. Thecatalyst was then pelletized, crushed and sieved to obtain catalystparticles sizes between 100 to 200 μm.

PREPARATION EXAMPLE 3: CHROMIA ON TITANIA

An aqueous solution of chromium nitrate (Cr(NO₃)₃ ·9H₂ O) (AlliedChemical Co.) was deposited onto TiO₂ (Degussa P25) as the support usingthe incipient wetness technique. As in the previous Examples, the TiO₂support (˜10% rutile and ˜90% anatase) was calcined in air at 500° C.and cooled to room temperature before impregnation with the chromiumprecursor. The support possessed a surface area of ˜55 m² /g. Afterimpregnation, the wet samples were dried at room temperature for 16hours, further dried at 110-120° C. for 16 hours and calcined at 450° C.for 13 hours. The catalyst was then pelletized, crushed and sieved toobtain catalyst particles sizes between 100 to 200 μm.

PREPARATION EXAMPLE 4: RHENIUM OXIDE ON TITANIA

An aqueous solution of perrhenic acid (HReO₄) (Aldrich) was depositedonto TiO₂ (Degussa P25) as the support using the incipient wetnesstechnique. As before, the TiO₂ support (˜10% rutile and ˜90% anatase)was calcined in air at 500° C. and cooled to room temperature beforeimpregnation with the rhenium oxide precursor. The support possessed asurface area of ˜55 m² /g. After impregnation, the wet samples weredried at room temperature for 16 hours, further dried at 110-120° C. for16 hours and calcined at 450° C. for 13 hours. The catalyst was thenpelletized, crushed and sieved to obtain catalyst particles sizesbetween 100 to 200 μm.

PREPARATION EXAMPLE 5: VANADIA ON ZIRCONIA

A vanadium oxide overlayer was deposited onto a zirconium oxide (ZrO₂)support (Degussa) having a surface area ˜39 m² g⁻¹ using an organicsolution of vanadium triisopropoxide oxide (Alfa, 95-98% purity). Inparticular, the vanadium overlayer was prepared by the incipient wetnessimpregnation method using a solution of vanadium triisopropoxide oxideand propanol-2 in a glove box filled with nitrogen. The solutions of thevanadium precursor and propanol-2 were thoroughly mixed with thezirconia support and dried at room temperature for 16 hours, furtherdried at 110-120° C. for 16 hours and calcined at 450° C. for 16 hours.The catalyst was then pelletized, crushed and sieved to obtain catalystparticles sizes between 100 to 200 μm.

PREPARATION EXAMPLE 6: VANADIA ON NIOBIA

A vanadium oxide overlayer was deposited on a niobia (Nb₂ O₅) support(55 m² g⁻¹) using vanadium triisopropoxide oxide (Alfa, 95-98% purity)and the incipient wetness technique. The niobia support was prepared bycalcining niobic acid (Niobia Products Co.) at 500° C. for two hours. Asolution of vanadium triisopropoxide oxide and propanol-2 was thoroughlymixed with the niobia support in a glove box filled with nitrogen, driedat room temperature for 16 hours, further dried at 110-120° C. for 16hours and calcined at 450° C. for 16 hours. The catalyst was thenpelletized, crushed and sieved to obtain catalyst particles sizesbetween 100 to 200 μm.

PREPARATION EXAMPLE 7: VANADIA ON ALUMINA

A vanadium oxide overlayer was deposited on an alumina (Al₂ O₃) support(Harshaw, 180 m² g⁻¹) using an organic solution of vanadiumtriisopropoxide oxide (Alfa, 95-98% purity) and the incipient wetnessimpregnation. A solution of the vanadium precursor and propanol-2 wasthoroughly mixed with the alumina support, in a glove box filled withnitrogen, dried at room temperature for 16 hours, further dried at110-120° C. for 16 hours and calcined at 500° C. for 16 hours. Thecatalyst was then pelletized, crushed and sieved to obtain catalystparticles sizes between 100 to 200 μm.

PREPARATION EXAMPLE 8: VANADIA ON SILICA

A vanadium oxide overlayer was deposited on an silica (SiO₂) support(Cab-O-Sil, 300 m² g⁻¹) using an organic solution of vanadiumtriisopropoxide oxide (Alfa, 95-98% purity) and the incipient wetnessimpregnation. A solution of the vanadium precursor and propanol-2 wasthoroughly mixed in a glove box filled with nitrogen with the SiO₂support, the wet silica was dried at room temperature for 16 hours,further dried at 110-120° C. for 16 hours and calcined at 500° C. for 16hours. The catalyst was then pelletized, crushed and sieved to obtaincatalyst particles sizes between 100 to 200 μm.

PREPARATION EXAMPLE 9: TUNGSTEN OXIDE ON SILICA

An aqueous solution of ammonium metatungstate (NH₄)₆ H₂ W₁₂ O₄₀ ·xH₂ O)(Pfaltz & Bauer, 99.9% purity) was deposited as an oxide overlayer ontoa silica (SiO₂) support (Cab-O-Sil, 300 m² g⁻¹) using the incipientwetness technique. After impregnation, the silica support was dried atroom temperature for 16 hours, further dried at 110-120° C. for 16 hoursand calcined at 500° C. for 16 hours. The catalyst was then pelletized,crushed and sieved to obtain catalyst particles sizes between 100 to 200μm.

PREPARATION EXAMPLE 10: NIOBIA ON SILICA

An aqueous solution of niobium oxalate Niobium Products Co.) wasdeposited onto a silica (SiO₂) support (Cab-O-Sil, 300 m² g⁻¹) using theincipient wetness technique. After impregnation, the silica support wasdried at room temperature for 16 hours, further dried at 110-120° C. for16 hours and calcined at 500° C. for 16 hours. The catalyst was thenpelletized, crushed and sieved to obtain catalyst particles sizesbetween 100 to 200 μm.

PREPARATION EXAMPLE 11: TITANIA ON SILICA

Titanium isopropoxide (Aldrich) in a toluene solution was impregnatedonto a silica (SiO₂) support (Cab-O-Sil, 300 m² g⁻¹) under a nitrogenblanket to form a titania overlayer using the incipient wetnesstechnique. After impregnation, the wet silica was dried at roomtemperature for 16 hours, further dried at 110-120° C. for 16 hours andcalcined at 500° C. for 16 hours. The catalyst was then pelletized,crushed and sieved to obtain catalyst particles sizes between 100 to 200μm.

PREPARATION EXAMPLE 12: VANADIA AND TUNGSTEN OXIDE ON TITANIA

A vanadia and tungsten oxide on titania catalyst was prepared by a twostep incipient wetness impregnation method. A vanadium oxide overlayerwas deposited first on the TiO₂ support using a solution of vanadiumtriisopropoxide oxide (Alfa, 95-98% purity) and propanol-2 by theincipient wetness impregnation method in a glove box filled withnitrogen. The solution of the vanadium precursor and propanol-2 werethoroughly mixed with the TiO₂ (Degussa P25) as the support. The TiO₂support (˜10% rutile and ˜90% anatase) was prepared by previouscalcination in air at 500° C. and cooled to room temperature beforeimpregnation with the vanadium oxide precursor. The support possessed asurface area of ˜55 m² /g. After impregnation, the wet TiO₂ was dried atroom temperature for 16 hours, further dried at 110-120° C. for 16 hoursand calcined at 450° C. for 12 hours. Subsequently, an aqueous solutionof ammonium metatungstate ((NH₄)₆ H₂ W₁₂ O₄₀ ·xH₂ O) was deposited as anoxide overlayer onto the TiO₂ support, again using the incipient wetnesstechnique. After impregnation, the wet samples were dried at roomtemperature for 16 hours, further dried at 110-120° C. for 16 hours andcalcined at 500° C. for 16 hours. The catalyst was then pelletized,crushed and sieved to obtain catalyst particles sizes between 100 to 200μm.

PREPARATION EXAMPLE 13: VANADIA AND TITANIA ON SILICA

A vanadia and titania on silica catalyst was prepared by a two stepincipient wetness impregnation method. The silica support used for thisstudy was Cabosil EH-5 (380 m² /g). This fluffy material was treatedwith water in order to condense its volume for easier handling. Then thewet SiO2 was dried at 120° C. and subsequently calcined at 500° C.overnight. The resulting surface area was 332 m² /g. This waterpretreatment did not change the dispersion ability of the silica, sincean isopropanol pretreated silica also resulted in the same surface areaand the same dispersion capacity. A titanium oxide overlayer wasdeposited first on the silica (SiO₂) support under a nitrogen blanketusing titanium isopropoxide (Aldrich) in a toluene solution by theincipient wetness impregnation method in a glove box filled withnitrogen. After impregnation, the loaded sample was dried at roomtemperature for 16 hours, further dried at 110-120° C. for 16 hours andcalcined at 500° C. for 4 hours. Subsequently, a solution of vanadiumtriisopropoxide oxide (Alfa, 95-98% purity) and propanol-2 wasimpregnated onto the silica (SiO₂) support containing titania againusing the incipient wetness technique. The solution of the vanadiumprecursor and propanol-2 was thoroughly mixed with the SiO₂ supportcontaining titania. After impregnation, the wet SiO₂ was dried at roomtemperature for 16 hours, further dried at 110-120° C. for 16 hours andcalcined at 450° C. for 2 hours. The catalyst was then pelletized,crushed and sieved to obtain catalyst particles sizes between 100 to 200μm.

The above-synthesized catalysts, as well as one other bulk metal oxidecatalyst, were examined for their ability to oxidize methyl mercaptansselectively to formaldehyde generally using the following equipment andmethods.

Catalytic Reactor

The oxidation reactions were carried out in an isothermal fixed-bedintegral mode reactor operating at atmospheric pressure. Themethanethiol (CH₃ SH) diluted in helium, was supplied by Scott SpecialtyGases. The reactant gas was further diluted in helium and air (BlueValley Welding Supply, total hydrocarbons concentration <1 ppm, H₂ Oconcentration <3 ppm) and sent to the reactor through glass tubingconnected with Teflon fittings. Flow rates and concentrations werecontrolled by two mass flow controllers (Brooks 5850 D, 1-100 sccm forhelium and Omega FMA-767-V, 0-1 slpm). The lines were heated to 70° C.for the methanethiol oxidation studies to prevent condensation. Thetotal gas flow was maintained between 150 and 200 ml/min. The reactorwas kept in a vertical position and made of 6-mm O.D. Pyrex glass.Heating tape was used in conjunction with a feedback temperaturecontroller (Omega CN 9000) to obtain the desired reactor temperature.The catalysts were held at the middle of the reactor tube between aporous glass frit, pore size of 40 to 60 μm, and a glass wool plug. Eachcatalyst sample was always pretreated by heating at 500° C. for 2 to 3hours in flowing air, to remove adsorbed water on the catalyst surfaceprior to initiation of an experiment. The outlet of the reactor wasconnected to an FTIR cell (Infrared Analysis, Inc; Model#G4-Tin-Ta-Ba-Ag), which was used to analyze the reaction products. Thelines between the outlet and the cell were heated to avoid condensationof the products. The flow of reaction products sent to the FTIR cell wascontrolled by a needle valve (Nupro Company, SS-4BRG).

Composition Analysis by FTIR

Analysis of the reaction products was accomplished using a Midac Inc.FTIR, (model #101250, series 2-4). Samples were analyzed in a path gascell (Infrared Analysis, Inc; Model #G-4-Tin-Ta-Ba-Ag), which has aneffective length of 10 m and a volume of 3.1 L. The spectrometer wascontrolled by a microcomputer (Sprouse Scientific, model TECH- 1000 A)to provide acquisition and manipulation of the spectra: display,subtraction, zoom, etc. The spectra were obtained using 16 scans at aresolution of 0.5 cm⁻¹. The FTIR analysis required about 10 minutes.

Methanethiol oxidation was investigated with a variety of supportedmetal oxide and bulk metal oxide catalysts as follows:

EXAMPLE 1

In a series of experiments, a supported oxide catalyst prepared inaccordance with Preparation Example 1, comprising about 1% vanadia (V₂O₅) on titania (TiO₂) catalyst, was contacted with a nitrogen streamcontaining methanethiol over a wide temperature range in order tooptimize the formation of formaldehyde. Mercaptan conversions weremeasured by both increasing and decreasing the temperature between 200and 450° C., and no temperature hysteresis was observed. The reactionproducts of this methanethiol oxidation over the 1% V₂ O₅ /TiO₂ catalystas a function of temperature is graphically presented in FIG. 1. Asillustrated, formaldehyde was found to be the predominant product. Inthese tests, dimethylthiomethane (H₂ C(SCH₃)₂) was observed as anintermediate between 200 to 300° C., and dimethyl disulfide (CH₃ S)₂ wasfound as an intermediate between 300 to 400° K. Carbon monoxide andcarbon dioxide appeared in small amounts as reaction products; but theformation of CO increased at elevated temperatures. Sulfur dioxideproduction tracked the formation of formaldehyde.

EXAMPLES 2-16

Using substantially the same equipment and procedures as Example 1, avariety of both metal oxide supported catalysts and a bulk metal oxidecatalyst were tested for their ability to oxidize methanethiolselectively to formaldehyde. While the majority of the data wereobtained at a reaction temperature of 350° C., Examples 8, and 12-14were run at 400° C., since formaldehyde was not detected in the productusing these catalysts at 350° C. The feed gas contained 1150 ppm ofmethanethiol and was introduced into the reactor at a volumetric flowrate of 150 ml/min. The iron-molybdate catalyst contained iron (Fe₂ O₃)and molybdenum (MoO₃) in a molar ratio (Fe:Mo) of 1.0/2.15 and wasobtained from Perstorp. The results of these tests are reported in Table1.

It will be understood that while the invention has been described inconjunction with specific embodiments thereof the foregoing descriptionand examples are intended to illustrate, but not limit the scope of theinvention. Other aspects, advantages and modifications will be apparentto those skilled in the art to which the invention pertains, and theseaspects and modifications are within the scope of the invention, whichis limited only by the appended claims.

                                      TABLE 1                                     __________________________________________________________________________                            Catalyst                                                                           Conversion                                                                           Selectivity (mol %)                                       Preparation                                                                           Load of CH.sub.3 SH Carbon                              Example Catalyst Example (mg) (Mole %) Formaldehyde Monoxide Carbon                                                                    Dioxide            __________________________________________________________________________                                                               COS                2     1.15%V.sub.2 O.sub.5 /TiO.sub.2                                                         1       10   54     78      11    3        8                    3 1%V.sub.2 O.sub.5 /TiO.sub.2 1A 10 48 84 10 4 2                             4 1%MoO.sub.3 /TiO.sub.2 2 100 84 79 12 9 0                                   5 1%CrO.sub.3 /TiO.sub.2 3 100 79 81 12 7 0                                   6 1%Re.sub.2 O.sub.7 /TiO.sub.2 4 10 80 76 18 3 2                             7 Fe.sub.2 (MoO.sub.4).sub.3 + -- 100 40 85 10 5 0                             MoO.sub.3                                                                    8 1%V.sub.2 O.sub.5 /ZrO.sub.2 5 10 57 89 8 2.6 0.4                           9 1%V.sub.2 O.sub.5 /Nb.sub.2 O.sub.5 6 20 45 72 21 7 0                       10 1%V.sub.2 O.sub.5 /Al.sub.2 O.sub.3 7 100 61 37 16 6 41                    11 1%V.sub.2 O.sub.5 /SiO.sub.2 8 100 63 84 9 7 0                             12 1%WO.sub.3 /SiO.sub.2 9 100 46 49 11 4 36                                  13 2.5%Nb.sub.2 O.sub.5 /SiO.sub.2 10  100 42 50 13 2 35                      14 10%TiO.sub.2 /SiO.sub.2 11  100 45 65 16 3 16                              15 1%V.sub.2 O.sub.5 /7%WO.sub.3 / 12  10 52 82 14 4 0                         TiO.sub.2                                                                    16 10%V.sub.2 O.sub.5 / 13  100 71 85 8 7 0                                    15%TiO.sub.2 /SiO.sub.2                                                    __________________________________________________________________________

What is claimed is:
 1. A process for producing formaldehyde and methylmercaptans from methane (CH₄) and hydrogen sulfide (H₂ S) comprisingpartially oxidizing CH₄ to formaldehyde and carbon oxides over a firstcatalyst, recovering the formaldehyde separate from the carbon oxides,contacting the separated carbon oxides and the H₂ S with a secondcatalyst comprising a supported metal oxide of a metal selected from thegroup consisting of titanium (Ti), zirconium (Zr), molybdenum (Mo),rhenium (Re), vanadium (V), chromium (Cr), tungsten (W), manganese (Mn),niobium (Nb), tantalum (Ta) and mixtures thereof, wherein said supportis selected from titania, silica, zirconia, alumina, ceria, magnesia,niobia, lanthanum oxide, tin oxide and mixtures thereof with the provisothat the support and the supported metal are not the same, to convertsaid carbon oxides and H₂ S to methyl mercaptans.
 2. A process forproducing formaldehyde and methyl mercaptans from a sour natural gasstream containing methane (CH₄) and hydrogen sulfide (H₂ S) comprisingseparating H₂ S from the sour natural gas, partially oxidizing CH₄recovered from the sour natural gas to formaldehyde and carbon oxidesover a first catalyst, recovering the formaldehyde separate from thecarbon oxides, contacting the separated carbon oxides and H₂ S with asecond catalyst comprising a supported metal oxide of a metal selectedfrom the group consisting of titanium (Ti), zirconium (Zr), molybdenum(Mo), rhenium (Re), vanadium (V), chromium (Cr), tungsten (W), manganese(Mn), niobium (Nb), tantalum (Ta) and mixtures thereof wherein saidsupport is selected from titania, silica, zirconia, alumina, ceria,magnesia, niobia, lanthanum oxide, tin oxide and mixtures thereof withthe proviso that the support and the supported metal are not the same,to convert said carbon oxides and H₂ S to methyl mercaptans.
 3. Aprocess for producing formaldehyde and methyl mercaptans from methane(CH₄) and hydrogen sulfide (H₂ S) comprising partially oxidizing CH₄ toformaldehyde and carbon oxides over a first catalyst, recovering theformaldehyde separate from the carbon oxides, contacting the separatedcarbon oxides and the H₂ S with a second catalyst comprising a supportedmetal oxide of a metal selected from the group consisting of titanium(Ti), molybdenum (Mo), rhenium (Re), vanadium (V), chromium (Cr),tungsten (W), manganese (Mn), niobium (Nb), tantalum (Ta) and mixturesthereof, wherein said support is selected from titania, zirconia,niobia, ceria, tin oxide and mixtures thereof with the proviso that thesupport and the supported metal are not the same, to convert said carbonoxides and H₂ S to methyl mercaptans.
 4. A process for producingformaldehyde and methyl mercaptans from a sour natural gas streamcontaining methane (CH₄) and hydrogen sulfide (H₂ S) comprisingseparating H₂ S from the sour natural gas, partially oxidizing CH₄recovered from the sour natural gas to formaldehyde and carbon oxidesover a first catalyst, recovering the formaldehyde separate from thecarbon oxides, contacting the separated carbon oxides and H₂ S with asecond catalyst comprising a supported metal oxide of a metal selectedfrom the group consisting of titanium (Ti), molybdenum (Mo), rhenium(Re), vanadium (V), chromium (Cr), tungsten (W), manganese (Mn), niobium(Nb), tantalum (Ta) and mixtures thereof, wherein said support isselected from titania, zirconia, niobia, ceria, tin oxide and mixturesthereof with the proviso that the support and the supported metal arenot the same, to convert said carbon oxides and H₂ S to methylmercaptans.
 5. The process of claim 3 or 4 wherein the methyl mercaptansare contacted with a third catalyst selected from a supported metaloxide catalyst and a bulk metal oxide catalyst under oxidizingconditions for a time sufficient to convert at least a portion of themethyl mercaptans to formaldehyde and sulfur dioxide, and recoveringsaid formaldehyde.
 6. The process of claim 5 wherein the supported metaloxide third catalyst has a metal oxide overlayer of a metal selectedfrom the group consisting of titanium (Ti), zirconium (Zr), molybdenum(Mo), rhenium (Re), vanadium (V), chromium (Cr), tungsten (W), manganese(Mn), niobium (Nb), tantalum (Ta) and mixtures thereof.
 7. The processof claim 6 wherein the supported metal oxide third catalyst has a metaloxide support selected from the group consisting of titania, silica,zirconia, alumina, niobia, magnesia, ceria, lanthanum oxide, tin oxideand mixtures thereof.
 8. The process of claim 7 wherein the loading ofthe metal oxide of vanadium (V), niobium (Nb), molybdenum (Mo), chromium(Cr), rhenium (Re), titanium (Ti), tungsten (W), manganese (Mn),tantalum (Ta) and mixtures thereof on the support of the second catalystranges between about 0.5 to 30 percent by weight of the second catalystweight and the metal oxide overlayer of the supported metal oxide thirdcatalyst comprises 0.5 to 35 percent by weight of the supported metaloxide second catalyst.
 9. The process of claim 8 wherein the supportedmetal oxide third catalyst is selected from the group consisting of avanadia overlayer on a titania support, a molybdenum oxide overlayer ona titania support, a chromium oxide overlayer on a titania support, arhenium oxide overlayer on a titania support, a vanadia overlayer on azirconia support, a vanadia overlayer on a niobia support, a vanadiaoverlayer on an alumina support, a vanadia overlayer on a silicasupport, a tungsten oxide overlayer on a silica support, a niobiaoverlayer on a silica support, and a titania overlayer on a silicasupport.
 10. The process of claim 9 wherein the second catalyst and thesupported metal oxide third catalyst each comprises a vanadia overlayeron a titania support, wherein the vanadia is present in an amount of 1to 15% by weight of said catalysts.
 11. The process of claim 5 whereinthe bulk metal oxide catalyst is selected from the group consisting ofmolybdates (Mo), chromates (Cr), vanadates (V), rhenates (Re), titanates(Ti), niobates (Nb), tungstates (W) and mixtures thereof.
 12. Theprocess of claim 11 wherein the bulk metal oxide catalyst comprises atleast one member selected from the group consisting of PbV₂ O₆, NaVO₃,Na₃ VO₄, BiVO₄, AlVO₄, FeVO₄, Mg₃ (VO₄)₂, Mg₂ V₂ O₇, CeVO₄, Zn₃ (VO₄)₂,CdV₂ O₇, Zn₂ V₂ O₇, VOPO₄, KVO₃, Pb₂ V₂ O₇, TlVO₄, PbMoO₄, CaMoO₄, Bi₂Mo₂ O₉, Bi₃ (FeO₄)(MoO₄)₃, Na₂ MoO₄, MnMoO₄, Gd₂ (MoO₄)₃, MgMoO₄,CuMoO₄, CoMoO₄, Fe₂ (MoO₄)₃, Te₂ MoO₇, CoMoO₄, Al₂ (MoO₄)₃, Cr₂ (MoO₄)₃,Na₂ Mo₂ O₇, YNbO₄, YbNbO₄, LiNbO₃, NaNbO₃, KNbO₃, AlNbO₄, K₈ Nb₆ O₁₉,BiNbO₄, SbNbO₄, NbOPO₄, CaNb₂ O₆, K₄ Nb₆ O₁₇, KCa₂ Nb₃ O₁₀, Li₆ WO₆,FeWO₄, CoWO₄, MnWO₄, NiWO₄, CuWO₄, CaWO₄, Cs₂ WO₄, Na₂ WO₄, BaWO₄, Fe₂(WO₄)₃, Al₂ (WO₄)₃, SrWO₄, K₂ WO₄, Na₂ W₂ O₇, Li₂ WO₄, CsLuW₂ O₈, BiWO₄,Na₂ CrO₄, Na₂ Cr₂ O₇, Na₂ Cr₃ O₁₀, Na₂ Cr₄ O₁₃, K₂ CrO₄, K₂ Cr₂ O₇, K₂Cr₃ O₁₀, K₂ Cr₄ O₁₃, Fe₂ (CrO₄)₃, CaCrO₄, Cs₂ CrO₄, BiCrO₄, NaReO₄, Li₆ReO₄, Mg(ReO₄)₂, Na₂ TiO₄, NaTiO₃, BaTiO₄, and BaTiO₃.
 13. The processof claim 3 or 4 wherein said contacting with the second catalyst isconducted at a temperature between 225° and 450° C.
 14. The process ofclaim 5 wherein said contacting with the second catalyst is conducted ata temperature between 225° and 450° C. and said contacting with thethird catalyst is conducted at a temperature between 200° and 700° C.15. The process of claim 9 wherein said contacting with the secondcatalyst is conducted at a temperature between 250° and 400° C. and saidcontacting with the third catalyst is conducted at a temperature between325° and 500° C.
 16. The process of claim 15 wherein said gas containingsaid methyl mercaptan is contacted with said third catalyst such thatbetween 10⁻² and 10⁴ cubic centimeters of methyl mercaptan contacts agram of catalyst per minute.
 17. The process of claim 16 wherein between0.1 and 100 cubic centimeters of methyl mercaptan contact a gram ofcatalyst per minute.
 18. The process of claim 3 or 4 wherein said carbonoxides and H₂ S are converted to the following compounds CH₃ SH, CH₃SCH₃, CH₃ SSCH₃ or mixtures thereof.